1. Field of the Invention
The present invention relates to an image reading apparatus, and particularly to an image reading apparatus capable of reading a front side and a back side of a document sheet simultaneously or almost simultaneously.
2. Related Background Art
Heretofore, when a both-side document sheet is read, a document sheet reversing mechanism is provided inside an ADF, and a front side of the document sheet is first read on the side of an image reading apparatus, and then the document sheet is, after having been read, reversed by the document sheet reversing mechanism inside the ADF, and then a back side of the document sheet is read on the side of the image reading apparatus, thereby reading a both-side document sheet image in accordance with this operation.
However, in this configuration, the time for reversing the document sheet is a dead time for the image reading apparatus, so that reading performance is lowered. Further, the document sheet is often damaged because it is reversed.
In view of this problem, a one-pass both-side image reading apparatus has recently been proposed which is equipped with a reading system inside an ADF and reads a front side image and a back side image by one-time document sheet conveying operation. FIG. 6 is a view showing a schematic configuration of the one-pass both-side image reading apparatus. A reading operation of the one-pass both-side image reading apparatus will be described using FIG. 6.
A plurality of both-side document sheets disposed on a document sheet putting table 71 of an ADF 70 is conveyed one by one. In a conveying path, the document sheets are separately conveyed one by one by means of separating rollers 72 and 73, and the document sheets are conveyed along conveying guides 74 and 77 in accordance with the turning in a direction of an arrow of a conveying roller 75.
Before the document sheet is conveyed to a reading position, a reading unit 200 moves right under a shading corrector plate 211 and performs shading correction. After finishing the shading correction, the reading unit 200 moves right under a viewer window 212, and is in a read standby state until the document sheet is conveyed.
When the document sheet is conveyed, the front side image of the document sheet is read right under the conveying roller 75. More specifically, a line image of the document sheet image illuminated by a lamp 201 inside the image reading apparatus 200 is focused on a CCD 209 via a first mirror 205, a second mirror 206, a third mirror 207 and a lens 208, and the line image is read. The conveying roller 75 conveys the document sheets at a predetermined speed, and the document sheets are sequentially read in a sub-scanning direction, whereby the front side image of the document sheet is read.
When reading of the front side of the document sheet image progresses and an edge of the document sheet is conveyed right under an equal size reading box 250, an image of the document sheet back side is read by the equal size reading box 250. An optical system of the equal size reading box 250 is constituted of a Selfoc lens 251, an image sensor 252 and a lamp 253, and the line image of the document sheet back side image illuminated by the lamp 253 is focused on the image sensor 252 via the Selfoc lens 251. The line image focused on a contact sensor is an image of an equal size. Since the conveying roller 75 conveys the document sheets at a predetermined speed, reading of the document sheet in the sub-scanning direction is sequentially performed, and thus the back side image of the document sheet is read. The both-side document sheet having been read is sequentially discharged at a discharge tray 78.
Image data of the front side and back side thus read is sent to an unillustrated image processing circuit, and the images are reproduced by a host computer, a printer or the like connected to the image reading apparatus, and then image formation processing is applied so that the images are formed on a display or a printing paper.
As described above, in case the image is copied by the printer to which a reader for simultaneously reading the both-side document sheet is connected, since both-side images can not be printed simultaneously on a printer side even if the reader simultaneously reads the both-side images. Therefore, the reader can not send the image data of the back side until the front side has been printed if a image data bus width is the same as the bus width for one-side document sheet reading. Plus such a method has to be applied that the image data of the back side document sheet is stored in a buffer memory having about a one-side capacity, and after the data of the front side has been sent, the image data of the back side document sheet stored in the buffer memory is sent. For this reason, there is caused a big problem of increased costs for the buffer memory. To solve this problem, double system data buses may be provided for sending front side image data and for sending back side image data, but it is not realistic because a cable which connects the reader and the printer becomes extremely thick considering recent increasing bits of image data.
Furthermore, since the printer sequentially prints the back side image followed by the front side image, the reader has to be on standby for almost a printing time of one-side amount until the next document sheet reading starts after the first both-side document sheet has been read, and thus the reader has not been able to exert its performance of simultaneously reading the both sides.